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8985 



Bureau of Mines Information Circular/1984 




Pickling of Stainless Steels— A Review 



By Bernard S. Covino, Jr., John V. Scalera, 
and Philip M. Fabis 




UNITED STATES DEPARTMENT OF THE INTERIOR 



■" a' 11 * 






'.afaw^iiwrewsawragpfflp 



Information Circular 8985 



Pickling of Stainless Steels— A Review 



By Bernard S. Covino, Jr., John V. Scalera, 
and Philip M. Fabis 



' ■"■ •:• ■ 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 




Library of Congress Cataloging in Publication Data: 




V 



<& 






<* 



Covino, B. S. (Bernard S.) 

Pickling of stainless steels— a review. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8985) 

Bibliography: p. 13-15. 

Supt. of Docs, no.: I 28.27:8985. 

1. Steel, Stainless— Pickling. I. Scalera, John V. II. Fabis, Philip 
M. III. Title. IV. Series: Information circular (United States. Bu- 
reau of Mines) ; 8985. 



TN295.U4 [TS654] 622s [669'. 142] 84-600164 



k 



4 

£ CONTENTS 

Page 

i 

V Abstract 1 

j Introduction 2 

/ Effect of hot and cold working on pickling 3 

Hot working 3 

Cold working 4 

Effect of annealing on pickling 4 

•XEf f ect of conditioning on pickling 6 

Degreas ing 6 

Abrasive blasting 6 

^ Chemical conditioning 6 

N. Reducing acids 7 

Qv/N Oxidizing acids 7 

Electrolytic acid conditioning 7 

r Anodic conditioning « ... . rf 7 

v~/ Cathodic conditioning '.'. .'..*.. 8 

Alternating current conditioning ' 8 

Electrolytic neutral conditioning 8 

Salt bath conditioning 8 

Reducing bath 9 

Oxidizing bath 9 

Electrolytic bath 9 

Effect of pickling bath variables on pickling 10 

Pickling operation 10 

Bulk alloy dissolution 11 

Solution composition 11 

Acid concentration 11 

Mixed acids 11 

Temperature 12 

Chromium-depleted zone dissolution 12 

Research needs 12 

References 13 

ILLUSTRATION 

/^l. Schematic of steps involved in processing stainless steels 2 

\ 



9\ 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


A/ft 2 


ampere per square foot 


ym/yr 


micrometer per year 


°C 


degree Celsius 


rpm 


revolution per minute 


min 


minute 


vol pet 


volume percent 


Um 


micrometer 


wt pet 


weight percent 



PICKLING OF STAINLESS STEELS-A REVIEW 

By Bernard S. Covino, Jr., John V. Scalera, and Philip M. Fabis 




ABSTRACT 

The Bureau of Mines is conducting a study of methods to improve the 
efficiency of the process used to pickle stainless steels. A review of 
the literature on pickling of stainless steels showed that the chemistry 
of several process operations involved in the pickling of stainless 
steels is not fully understood, and that further research could improve 
the pickling efficiency. The benefits of this research would be a re- 
duction in the annual loss of several thousands of tons of critical 
minerals such as nickel and chromium, and a reduction in the amount of 
solids and spent acid solution that are currently discarded. The con- 
clusion from this review is that further research is needed in four 
operations that either directly or indirectly influence the pickling 
process: hot working, annealing, conditioning, and the actual operation 
of pickling. Laboratory studies of the pickling operation are presently 
in progress. 

'Research chemist. 
^Materials engineer. 
Avondale Research Center, Bureau of Mines, Avondale, MD. 



INTRODUCTION 



The various operations involved in pro- 
cessing stainless steels are given in 
figure 1. After being worked by hot or 
cold rolling, the steel is softened by 
annealing. Oxide forms on the stainless 
steels during this annealing process, 
which, as shown in figure 1, occurs sev- 
eral times. Conditioning is used to 
facilitate the pickling process. Mixed- 
acid pickling, or pickling by a solution 
of two or more acids , is then used for 
cleaning the oxide-covered stainless 
steels. In addition to removing the an- 
nealing scale, pickling also removes a 
very thin (1- to 5-pm) region depleted in 
chromium between the oxide and the bulk 
stainless steel. Loss of chromium and 
nickel from this region and the oxide is 
inherent in this part of the pickling op- 
eration. In practice, the stainless 
steel may be left in the pickling solu- 
tion longer than necessary, causing ex- 
cessive dissolution of the bulk steel, 
resulting in losses of several thousand 
tons of chromium and nickel annually. 
The combined dissolution products can 
build up to a point where the action at 
the pickling bath stops, resulting in a 
sizable disposal problem when the bath is 
replaced. The dissolved metals also in- 
crease the use of acids in the pickling 
bath by complexing or precipitating acid 
salts. The need to study the pickling 
process was formulated during discussions 
between the Bureau of Mines and the Amer- 
ican Iron and Steel Institute (AISI). 
Both groups concluded that the problems 
of loss of critical minerals, excess use 
of acids , and disposal of spent solutions 
could be lessened by a better understand- 
ing of the entire pickling process. 

The literature pertinent to the pick- 
ling of austenitic stainless steels was 
reviewed. Data bases such as Chemical 
Abstracts, Metadex, Compendex, and NTIS 
(National Technical Information Service) 
were searched from 1900 to 1983 where 
applicable. Although articles in all 
languages were accepted in the search, 
the review was done mainly on articles in 
English. The general purpose of the re- 
view was to assess the technology of 
stainless steel pickling and to present a 



critical examination of the mechanism of 
mixed-acid pickling of stainless steels 
in terms of all the important process 
operations and operating parameters. 

All such operations and parameters are 
considered in light of how much knowledge 
is available and what further knowledge 
is necessary for a better understanding 
and control of the pickling process. 
This review begins by considering those 
factors in the metal-forming operation 
that can affect subsequent pickling be- 
havior. The effect of annealing on pick- 
ling is then addressed. Conditioning 
treatments prior to pickling have a very 



Ingot 




Hot work 






Anneal 






















■ 


j 








Conditioning: 

mechanical 

or 

chemical 






1 


j 






Pickling 






" 






Cold work 












' 






Anneal 






1 








Conditioning: 
chemical 

or 
electrolytic 

or 
salt bath 








• 






Pickling 


As necessary 










' 






Finished 
material 





FIGURE 1. - Schematic of steps involved in process- 
ing stainless steels. Indicated steps are repeated as 
often as necessary to thin the material to the desired 
thickness. 



significant effect on pickling and are 
addressed next. Finally, the operation 
of the pickling bath is considered. The 



present understanding of the mechanism of 
pickling of stainless steel is developed 
in detail in this last section. 



EFFECT OF HOT AND COLD WORKING ON PICKLING 






In order to be inclusive, the effects 
of hot and cold working on the pickling 
of stainless steels are briefly consid- 
ered. Working of the stainless steel 
usually has no direct effect on the pick- 
ling operation because an annealing op- 
eration is interposed between the working 
and the pickling steps. However, some 
effects of hot and cold working can in- 
fluence the annealing operation or remain 
unchanged after the annealing operation. 
Of the two types of working, hot working 
presents the greatest potential problem 
because of its ability to change the 
chemistry and grain structure of the 
stainless steel, resulting in subsequent 
changes in the scale formed during 
annealing. 



HOT WORKING 



Hot working, the process of mechanical 
deformation of a material at temperatures 
above its recrystallization temperature, 
can significantly alter the grain size of 
metals. The degree to which this altera- 
tion occurs for alloys such as stainless 
steels depends on the degree of deforma- 
tion, the number and frequency of defor- 
mation steps, and the initial and final 
working temperatures. The degree of 
deformation determines the stored energy 
in a material that is the driving force 
for recrystallization to occur. The fre- 
quency of deformation steps and initial 
and final temperatures determines the 
rate of crystallization and the occur- 
rence and rate of grain growth. Both a 
lower deformation temperature and a 
greater amount of deformation produce a 
smaller ultimate grain size. It is 
usually the temperature at which hot 
working is completed, the finishing tem- 
perature, that determines the average 
grain size (1_). 3 

^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



When hot working operations are fol- 
lowed by an anneal, the major effect of 
this altered grain size is on the result- 
ing annealing scale. If the mill scale 
conforms to the morphology of the metal 
surface, then a fine-grained scale forms 
on a fine-grained metal and a larger 
grained scale forms on a large-grained 
metal. Recent research (2) has shown 
that the scale formed on smaller- 
grain-size stainless steels contains more 
chromium than that on larger-grain-size 
steels. Oxide films containing more 
chromium could significantly affect the 
rate of pickling and possibly cause com- 
positional changes, such as chromium de- 
pletion, in the metal adjacent to the 
oxide. 

Severe problems can occur after hot 
working if there is significant chemical 
inhomogeneity or large quantities of in- 
clusions in the initial ingot. Regions 
of the ingot depleted of an alloy con- 
stituent could be smeared in the rolling 
direction, resulting in bands of differ- 
ent composition (3). Another phenomenon, 
similar in appearance to banding, is 
fiber. This elongated structure consists 
of nonmetallic inclusions which are elon- 
gated as the steel is worked (4^) . While 
the spontaneous recrystallization that 
occurs during hot working is usually una- 
ble to affect these localized composition 
changes , annealing usually eliminates 
them. If these variations in metal com- 
position are still present after the an- 
nealing step, however, preferential at- 
tack of the banded regions or pitting 
near the fibers could result in an ir- 
regular surface morphology. Since stain- 
less steel chemistry is closely con- 
trolled, these structures are rarely seen 
in commercial stainless steels; if pre- 
sent, they would be expected to signifi- 
cantly affect the pickling process. 

Sensitization to intergranular corro- 
sion may occur in austenitic stainless 



steels when they are subjected to a 
working temperature range of 450° to 
900° C or when the steel is slow-cooled 
from 1,050° C (_5_ ) . A chromium-depletion 
theory proposes that the precipitation of 
M23C6 carbides along a grain boundary 
results in a region depleted of chromium 
adjacent to the carbides. In some cases 
of very high purity (low-carbon) alloys, 
although there is no detectable carbide 
precipitation, a solute segregation 
theory proposes that a chromium-depleted 
region still exists (6) . The resistance 
to the pickling solution is greatly re- 
duced in these regions. Essentially, two 
dissimilar metals are in contact and an 
unfavorable anode-cathode area ratio is 
present (7^, pp. 35-36). In this case, 
the depleted zone sets up active-passive 
cells with large-area alloy grains acting 
as cathodes in contact with material in 
the grain boundaries of limited area act- 
ing as anodes. In addition, the grain 
boundary carbides can be susceptible to 
the mixed acid solution, and their dis- 
solution would produce further surface 



degradation. As with the banding and 
fiber phenomena, however, any sensitiza- 
tion should be removed in a proper an- 
nealing step, making hot-work-induced 
sensitization a minor problem area. 

COLD WORKING 

Cold working increases the stored 
energy of a material and, when coupled 
with an annealing process , can cause 
grain size changes similar to those dis- 
cussed for hot working. But, unless a 
deformation-induced segregation or trans- 
formation occurs that brings dissimilar 
concentrations of atoms into contact (8) , 
cold working has no other effect on the 
pickling behavior of properly annealed 
stainless steels. Simultaneous deforma- 
tion and pickling never occur in the in- 
dustrial processing scheme of stainless 
steels; therefore, cold working is im- 
portant only if new phases or the segre- 
gation of alloyed components result from 
the plastic deformation. 



EFFECT OF ANNEALING ON PICKLING 



Compared to hot and cold working, an- 
nealing probably has a more significant 
effect on the pickling rate of stainless 
steels than any other process preceding 
pickling. Annealing can be thought of as 
a relatively uncontrolled oxidation re- 
action; that is, the temperature can be 
controlled fairly accurately, but the 
atmosphere within the annealing furnace 
is usually not controlled. Also, from 
batch to batch of stainless steels, the 
heat-up time, time at temperature, and 
cool-down time are different for various 
metallurgical reasons. This variability 
in the annealing process can have a sig- 
nificant effect on the mill scale formed 
and thus on the ease of pickling of the 
stainless steel. Numerous investigations 
concerning the high-temperature oxidation 
of stainless steels in various atmos- 
pheres are available throughout the met- 
allurgical literature. Although there is 
considerable disagreement about the 
structure, thickness, and growth mechan- 
isms of the films, some agreement con- 
cerning film characteristics exists. 



Because of the short annealing times 
involved in stainless steel processing, 
the gaseous annealing environment reacts 
preferentially with the more reactive 
chromium component. This produces a re- 
gion of the base metal near the oxide 
that is significantly depleted in chromi- 
um. Consequently, the thin oxide films 
that form on these alloys during the ini- 
tial oxidation stages are composed mainly 
of Cr 2 3 with small amounts of Fe 2 3 or a 
spinel phase FeFe ( 2 _ x) Cr x 4 where 0<x<2 
(9-12) . These films are fairly adherent 
and protective, and since their ionic 
conductivity is low (13) , they prevent 
diffusive penetration of other ions and 
atoms through the scale. Although the 
Cr 2 3 films are adequate barriers, the 
iron oxide films are quite permeable, 
especially to carbon (14) . Under condi- 
tions of severe scaling, a stratified 
structure occurs that contains Cr 2 3 , 
FeFe (2 _ x )Cr x 4 , FeO, Fe 3 4 , and Fe 2 3 (8_, 
15). In certain cases, as in extended 
heating periods, the initially protective 
scales then become nonprotective. 



The scaling behavior of stainless 
steels under conditions similar to those 
in annealing furnaces has not been ade- 
quately studied. Brief exposures (1- to 
5-min time at temperature) in complex 
(0 2 , H 2 0, CO, C0 2 ) high-temperature (871° 
to 1,038° C) environments should be stud- 
ied. Complete characterization of the 
scales resulting from these exposures 
must be done in order to understand the 
chemistry and structure of the complex 
scales formed on stainless steels. It 
would then be necessary to correlate the 
ease of pickling of the steels with the 
chemistry and structure of the annealing 
scales, an area of research that could 
have significant impact on the pickling 
process as currently used. 

Because of the elevated-temperature 
anneal, three other metal-related phenom- 
ena that could have an impact on subse- 
quent pickling operations are embrittle- 
ment, sensitization, and a transformation 
to a dual-phase structure. 

Embrittlement (_1_) may occur upon heat- 
ing or slowly cooling certain ferritic 
and high-nickel austenitic steels through 
the 425° to 760° C temperature range. A 
precipitation reaction occurs in this 
temperature range producing a complex 
structure FeCr phase, the sigma (a) 
phase. Because this phase is rich in 
chromium, the presence of a chromium con- 
centration gradient between the a-phase 
and adjoining phases may affect the pick- 
ling behavior and corrosion resistance of 
stainless alloys (8) . With proper tem- 
perature control, however, this phase 
will not form. 

Elemental concentration gradients arise 
in the alloy owing to diffusional pro- 
cesses at elevated temperatures. For in- 
stance, if an 18Cr-8Ni stainless steel 
(i.e., 304) is heated to 1,150° C (a 
common heat-treatment temperature) and 
water-quenched, the single-phase austen- 
itic alloy is formed. The quenched-in 
austenitic structure is thermodynamically 
unstable at room temperature, but the 
transformation kinetics are extremely 
slow. If the same alloy, assuming a 
low carbon concentration, is heated to 
1,200° C and allowed to soak at that 



temperature for several hours , the FCC 
austenite partially transforms to fer- 
rite, a BCC structure. Therefore, a two- 
phase (austenite-ferrite) microstructure 
can be present. A problem arises in that 
the austenite phase contains more nickel 
and less chromium relative to the ferrite 
phase (8) . Thus , an elemental concentra- 
tion gradient is created between adjacent 
grains. The susceptibility of the alloy 
in this condition to localized corrosion 
in a mixed-acid pickling bath may be sig- 
nificantly enhanced. This enhancement is 
attributable to a galvanic effect arising 
between grains of different composition 
in close proximity. 

The final problem produced by elevated- 
temperature exposure of stainless steels 
is sensitization. Depending upon the 
carbon content of the alloy, precautions 
may have to be taken to avoid sensitizing 
conditions during annealing. As men- 
tioned previously, sensitization results 
in intergranular precipitation of M 2 3C 6 
carbides of high chromium content and 
chromium depletion of regions adjacent to 
these grain boundaries to below the 
necessary 12 wt pet required for stable 
passivity (_7, pp. 35-36). While this 
would cause an increased localized at- 
tack, sensitization is rarely a problem 
in the production of austenitic stainless 
steel. 

In summary, hot working and, to a small 
degree, cold working can have potentially 
deleterious effects on the pickling of 
stainless steels. Grain-size variations 
can cause variations in scale chemistry, 
and ingot inhomogeneity can cause banding 
and fibering. Sensitization can occur in 
both hot working and annealing, but em- 
brittlement usually occurs only during 
annealing. Typical annealing processes 
are poorly controlled, resulting in vari- 
ations in scale chemistry. All of these 
problems do not necessarily occur during 
present-day stainless steel processing, 
but there is a very real possibility for 
their occurrence. Therefore, in order to 
fully understand the pickling process and 
everything that affects it , the more im- 
portant of these problems (grain-size 
variations and annealing variations) 
should be studied further. 



EFFECT OF CONDITIONING ON PICKLING 



Conditioning of stainless steels is a 
process used to prepare annealed stain- 
less steel for the pickling process. Its 
purpose is to alter the annealing scale 
in order to reduce the time, temperature, 
and acid concentration used in the pick- 
ling process. The scale is either 
cracked by thermal or mechanical means , 
or constituents in the scale are chem- 
ically or electrolytically altered to In- 
crease their solubility in order to be 
able to reduce either the amount of HNO3 
and HF in the final pickling solution or 
the times and temperatures in the pick- 
ling process. 

In contrast to the well-understood 
mechanical conditioning process, the 
chemical and electrolytic conditioning 
techniques need to be studied further in 
order to understand exactly what chemical 
and structural changes result from the 
respective technique. 

DEGREASING 

Although degreasing is usually done be- 
fore annealing stainless steels, it also 
may be necessary, prior to a conditioning 
step, to remove surface contaminants such 
as oil, grit, graphite, metal chips, or 
other foreign matter that may have been 
transferred to the steel surface. De- 
pending on the type of foreign matter 
suspected on the surface of the steel, 
various techniques are employed. These 
techniques include the use of vapor de- 
greasing with organic solvents, water- 
soluble emulsifiers, chelates, and water- 
soluble alkaline cleaners (16) . Contam- 
inants on the surface of the heat-treated 
steel can often result in an oxidized 
grime during scale conditioning, which 
can severely inhibit the effectiveness of 
the subsequent pickling processes. Sur- 
face contamination also pollutes the 
pickling solutions, which reduces their 
effectiveness. 

Once the surface has been effectively 
cleaned, conditioning techniques help to 
oxidize, crack, and loosen scale to en- 
hance the effectiveness of final pickling 



operations. Depending upon the nature of 
the oxide film, various techniques are 
employed. Both mechanical and chemical 
techniques have been used successfully. 
The basic mechanical technique for scale 
conditioning is abrasive blasting. 

ABRASIVE BLASTING 

Abrasive blasting is one of the fastest 
of all conditioning techniques and has 
been successfully used in both batch and 
continuous on-line processing of stain- 
less steel strip (16-18) , although it is 
used almost exclusively on the latter. 
Abrasive blasting uses steel shot or sil- 
ica in sizes ranging from 100 to 2,500 
mesh depending on the material being con- 
ditioned. The abrasive is either dropped 
or directed by air pressure toward the 
surface of the steel at angles and ve- 
locities that allow the cracking, loosen- 
ing, and partial removal of heavy oxide 
scales without cold-working the steel's 
surface. The technique has limited ap- 
plication for batch-processing complex 
shapes because some oxidized areas not 
exposed to the shot blast are not totally 
conditioned. The use of carbon steel 
shot as an abrasive has been a topic of 
controversy (19) . Opponents feel that 
its use can embed iron particles in the 
surface of the stainless steel, resulting 
in future corrosion problems. This 
would, of course, depend on what treat- 
ments followed the carbon steel blasting. 
The abrasive silica used in blasting must 
be low in iron in order to avoid surface 
contamination. As with most mechanical 
descaling techniques, caution must be 
practiced to avoid work-hardening the 
steel surface. Heat-treated surfaces 
with thin oxide films , such as cold- 
rolled products, are usually not condi- 
tioned using mechanical techniques. 

CHEMICAL CONDITIONING 

The choice of chemical conditioning 
parameters varies significantly between 
stainless steel processing plants. Some 
of the parameters that must be taken into 
consideration are the nature of the base 



metal and oxide being removed, strip or 
batch processing, and chemical costs. 
For example, when working with an inter- 
mediate pickled 304 stainless steel, some 
companies use a salt bath conditioning 
process followed by an electrolytic 
nitric acid bath before the final HN0 3 -HF 
acid pickle; other processors go directly 
from the salt bath into the final pick- 
ling bath. The following paragraphs 
describe some of the more common chemical 
conditioning processes in use. 

Reducing Acids 

Reducing acid baths descale a metal by 
reducing the oxides in the scale and also 
liberate hydrogen at the oxide-metal in- 
terface. The most common reducing acids 
used in conditioning processes for an- 
nealed stainless steels are H 2 S0 4 and 
HC1. The specific acid solution used de- 
pends on the nature of the base material 
and oxide being treated; chemical costs, 
as well as the time permitted for condi- 
tioning, are usually determined by 
whether the metal is prepared by either 
continuous strip or batch processing. 
Solutions of 10 to 15 vol pet H 2 S0 4 at 
60° to 71° C are used for conditioning 
heavy oxides (20) . H 2 S0 4 solutions are 
relatively slow in comparison to solu- 
tions combining H 2 S0 4 (6 to 10 vol pet) 
with HC1 (6 to 10 vol pet) at 54° to 60° 
C (_21, pp. 684-689). Although HC1 is not 
as aggressive as H 2 S0 4 in attacking the 
base metal at elevated temperatures (77° 
to 93° C) , it is much more aggressive in 
attacking iron oxides than is H 2 S0 4 . The 
use of conditioning solutions containing 
HC1 requires critical control because 
ferric chlorides formed during the condi- 
tioning process can result in pitting of 
the base material ( 19 ) . Another condi- 
tioning solution that can cause base met- 
al pitting is H 2 S0 4 (8 to 11 wt pet) com- 
bined with NaCl (5 to 6 wt pet) at 60° to 
65° C U8). 

Oxidizing Acids 

In contrast to reducing acids , oxidiz- 
ing acids descale by oxidizing the scale 
to a higher oxidation state, thus in- 
creasing the solubility of the scale. 



HNO3 (8 to 10 vol pet at 38° to 54° C) is 
the most commonly used oxidizing acid 
(19) . Because of the greater cost of 
HNO3 compared to H 2 S0 4 , however, its use 
has been limited in the conditioning 
processes. 

ELECTROLYTIC ACID CONDITIONING 

Experimental studies on electrolytic 
pickling in acid solutions were described 
by Tamba, Azzerri, Bombara, and others 
(22-25). Industrial electrolytic condi- 
tioning using an H 2 S0 4 bath can be traced 
back to the 1920's and 1930' s where there 
was a need for faster pickling techniques 
to maximize output from on-line stainless 
steel strip processing (26) . 

The most common acids associated with 
electrolytic conditioning are H 2 S0 4 and 
HNO3. Electrolytic acid conditioning 
techniques are used presently on both 
martensitic and ferritic steels which, 
during the annealing process , have devel- 
oped tightly adhering thin oxide films 
(21 , pp. 689-690). Austenitic chromium- 
nickel steels can also be electrolytical- 
ly conditioned, although this condition- 
ing requires caution because of the 
greater potential for surface pitting. 
The probability of pitting is increased 
for thicker oxide scales because breaks 
in the oxide scale react more rapidly 
than scale-covered base metal. This 
highly localized activity can result in 
pitting before the bulk of the scale is 
removed . 

Anodic Conditioning 

There are three basic types of electro- 
lytic conditioning: anodic, cathodic, 
and alternating current (27) . Anodic 
conditioning either electrically oxidizes 
(dissolves) the surface or forces the 
surface into a passive state by an ap- 
plied anodic potential. When the surface 
is being electrically oxidized, the base 
metal is being attacked and the scale 
dislodges. When the surface is passive, 
then the base metal undergoes far less 
attack than the oxide scale, releasing 
oxygen gas at its surface. The oxygen 
mechanically agitates the solution, 



uplifts the surface scale, and can oxi- 
dize contaminants such as organic impuri- 
ties. An example of an anodic electro- 
lytic solution is 2 pet HN0 3 used at a 
current density of 75 to 200 A/ft 2 (21, 
pp. 689-690; 27_) . 

Cathodic Conditioning 

During cathodic electrolysis, the work- 
piece is charged to act as a cathode. 
The base metal is electrochemically pro- 
tected while the oxide scale is being re- 
duced. Cathodic electrolysis is faster 
than anodic; hydrogen gas generated on 
the metal surface helps to agitate the 
solution and lift off the oxides. In 
martensitic steels, however, this reac- 
tion can result in hydrogen embrittlement 
of the surface (27-28). 



solution is 10 vol pet H 2 S0 4 used at 88° 
C with a current density of 100 to 150 
A/ft 2 (27). 

Both ferritic and martensitic steels 
undergo annealing processes which form 
thin, tightly "skinned" oxides. Removal 
of these oxides by mechanical means could 
cold-work the surface. However, the use 
of reducing acids, such as H2SO4, which 
release hydrogen during the alternating 
current descaling process when the work- 
piece is the cathode, can be detrimental 
to martensitic steels. As in cathodic 
conditioning, these steels are subject to 
hydrogen embrittlement. Alternative con- 
ditioning techniques for martensitic 
steels include those in which the libera- 
tion of hydrogen on the steel surface is 
eliminated. 



Alternating Current Conditioning 

Very low frequency (one cycle per sev- 
eral minutes) alternating current elec- 
trolysis is used in conjunction with 
stainless strip processing where direct 
electrical contact to the workpiece is 
difficult. In fact, the current is usu- 
ally reversed only from tank to tank in a 
two-tank method, or, in a one-tank meth- 
od, it is reversed once in the tank. The 
basic reactions, however, are the same as 
described in anodic and cathodic condi- 
tioning. Operationally, the electrodes 
are placed above and below the strip, 
forming an anodic potential on one side 
of the strip and a cathodic potential on 
the other side (27) . During each alter- 
nating cycle, the polarity is reversed on 
the electrodes and, in turn, on the faces 
of the stainless steel strip. A varia- 
tion is the use of two tanks where the 
strip in the first tank is made cathodic 
with respect to the anodic electrode in 
the tank (26) . Here the base metal is 
electrochemically protected and hydrogen 
gas is formed to lift off the oxides. 
The strip then enters a second tank where 
it becomes anodic with respect to the ca- 
thodic electrode placed in the second 
tank. Here the surface and scale are ox- 
idized and release oxygen at the surface, 
which uplifts the scale. An example of 
an alternating current electrolysis 



ELECTROLYTIC NEUTRAL CONDITIONING 

Electrolytic neutral (the Ruthner 
"Neolyte" process) conditioning is simi- 
lar in mechanical design to electrolytic 
acid conditioning in that alternating 
cathodic and anodic electrodes are used 
to polarize the workpiece and induce oxi- 
dation and reduction of the surface scale 
(29) . The electrolytic neutral pickling 
involves a Na 2 S0 4 solution at tempera- 
tures of 65° to 85 C, resulting in safer 
operation conditions and reduced energy 
costs (29) . The final stages in the pro- 
cess result in a regeneration of the 
Na 2 S0 4 . 

SALT BATH CONDITIONING 

The use of both aqueous and molten salt 
baths as a pretreatment for acid pickling 
has been successful in increasing the 
ease of scale removal of all stainless 
steel types, permitting the use of re- 
duced acid concentration and decreasing 
the time of pickling needed to remove the 
scale Q6, JLjU 30-31). This reduction in 
acid concentration and pickling time re- 
duces the likelihood of hydrogen embrit- 
tlement in susceptible alloys. Other 
benefits of salt bath descaling are its 
fast process time and uniform surface 
finish (19). 



Salt bath treatments can be either re- 
ducing, oxidizing, or electrolytic (16, 
30 ) . Reducing and electrolytic baths 
have proven to be more effective in at- 
tacking heavy, tightly adhering oxide 
scales. 

Reducing Bath 

Reducing salt baths consist of a molten 
bath of NaOH (371° ±11° C) , containing 
1.5 to 2.0 wt pet sodium hydride (16, 
30 ) . The sodium hydride is formed in 
generators along the side of the descal- 
ing tank. Sodium and hydrogen gas react 
to produce the hydride, which must be 
constantly generated since the level of 
sodium hydride in solution is depleted 
during descaling. There are safety haz- 
ards involved in the use of sodium and 
hydrogen. Descaling takes place as the 
oxide films are reduced to a lower oxida- 
tion state: 

M 2 3 + xNaH = xNaOH + M 2 3 . x . 

After the workpiece is removed from the 
salt bath, it is water-quenched. This 
quenching thermally shocks the remaining 
oxide layer, fracturing and loosening it 
and making it more susceptible to the 
acid pickling. 

Oxidizing Bath 

As with the previously described caus- 
tic reducing salt bath, oxidizing salt 
baths also use molten NaOH (60 to 90 wt 
pet) at elevated temperatures (482° C) 
(16, 30) . Other constituents include 
sodium nitrate (7 to 32 wt pet) and so- 
dium chloride (1.5 to 6 wt pet). Some of 
the oxidizing reactions that take place 
are (31)-- 

2Fe0 + NaN0 3 = Fe 2 3 + NaN0 2 

Cr 2 3 + 3NaN0 3 = Cr 2 6 + 3NaN0 2 

2Ni0 + NaN0 3 = Ni 2 3 + NaN0 2 

C + 2NaN0 3 = C0 2 + 2NaN0 2 

As long as the bath is in contact with 
air, the NaN0 3 can be regenerated by the 
oxidation of the NaN0 2 formed. Trivalent 



chromium is oxidized to the hexavalent 
state, which readily dissolves into acid 
pickling solutions. 

Upon leaving the salt bath, the work- 
piece is quenched with water. The ther- 
mal shock involved in quenching the work- 
piece disrupts the oxidized scale. After 
the water quench, an acid pickling bath 
is needed to dissolve any remaining 
oxide. 

An aqueous oxidizing salt bath consists 
of NaOH (20 wt pet) and KMn0 4 (5 wt pet) 
(32) . This strong oxidizing solution may 
be used on all grades of stainless steel, 
especially where a light, tightly adher- 
ing scale has formed, such as is the case 
in cold rolling or in some controlled 
annealing atmospheres. The KMn0 4 -NaOH 
solution may also be used on heavier ox- 
ide scales. As with the other previously 
mentioned molten salt baths , the treat- 
ment is followed by acid pickling. 

Electrolytic Bath 

The use of an electrolytic technique in 
combination with molten salt baths has 
been applied successfully to continuous 
stainless steel strip processing lines. 
A typical bath consists of 75 pet NaOH, 
10 pet NaCl-NaF, 14 pet Na 2 C0 3 , and 1 pet 
other carbonates and is operated at 482° 
C. The salt baths are neither oxidizing 
nor reducing in nature until electrically 
polarized. Polarization of the workpiece 
is achieved by the use of a series of 
cathodes followed by anodes in the neu- 
tral salt bath (_16, 30). Opposite the 
cathode, the strip acts as an anode with 
oxidation taking place. As some of the 
scale is dissolved into solution, it 
reacts with the oxygen released during 
the oxidation process to form insoluble 
hydroxides. These hydroxides will not 
interfere with the surface of the steel 
strip as it becomes cathodic and under- 
goes reduction. The strip becomes ca- 
thodic as it passes below the anode. 
As reduction takes place at the scale- 
surface interface, hydrogen evolves, 
lifting the scale off the surface. After 
the sample passes through the salt bath, 
it is water-rinsed and acid-pickled for 
final scale removal and surface finish. 



10 



EFFECT OF PICKLING BATH VARIABLES ON PICKLING 



The pickling of stainless steels re- 
quires three distinct processes. The 
first process is the removal of the 
thermally grown oxide scale for appear- 
ance purposes and to facilitate further 
cold working of the steel. The second 
process maximizes the corrosion resist- 
ance of the final steel product by com- 
pletely dissolving the chromium-depleted 
zone that is generally formed during 
short high-temperature anneals in oxidiz- 
ing environments. The third process dis- 
solves the minimum amount of bulk steel 
necessary to give the desired whitening 
effect. These three processes occur, to 
some degree, simultaneously during the 
pickling operation and most probably are 
interdependent. Therefore, to understand 
the pickling operation, it is necessary 
to understand the effect of pickling bath 
variables, solution composition, and tem- 
perature on the pickling operation as a 
whole and on the dissolution of both the 
chromium-depleted region and the bulk 
steel. 

PICKLING OPERATION 

Before considering the effect of bath 
variables on the pickling of stainless 
steels, it is necessary to assess the 
state of understanding of the mechanism 
of pickling. It is generally agreed in 
the literature (24) that the pickling of 
stainless steels is accomplished by un- 
dercutting the oxide and that the high 
chemical reactivity of the chromium- 
depleted zone essentially controls the 
rate of the pickling operation. There 
were, however, no in-depth studies of the 
mechanism or even any proof found in the 
body of literature reviewed here. While 
it was difficult to determine how this 
mechanism was discovered, it is clear 
that little is known empirically about 
the mixed acid pickling of stainless 
steels. Other research indicates that 
dissolution of the oxide is inconsequen- 
tial in the actual removal of the oxide 
(23) . What appears to be important is 
that the oxide be sufficiently disrupted 



to permit the penetration of the pickling 
solution. Most changes in oxide porosity 
occur during the conditioning process. 

The ideal pickling solution is one that 
will easily penetrate the thermally grown 
oxide, rapidly dissolve the chromium- 
depleted zone, and dissolve only a small 
layer of the bulk steel. This solution 
would optimize the rate of pickling while 
minimizing the unnecessary and excessive 
loss of bulk alloy. The most commonly 
used pickling solution for austenitic 
stainless steels is a mixed acid consist- 
ing of HN0 3 and HF in various propor- 
tions. By varying the ratio of HN0 3 to 
HF, it is possible to pickle different 
types of austenitic steels. This solu- 
tion accomplishes all three processes 
listed above and provides a very white 
surface. There are, however, potential 
problems of overpickling the steel, re- 
sulting in excessive grain boundary at- 
tack. This could affect the reflectivity 
of the surface and decrease the corrosion 
resistance. This mixed acid is also ex- 
pensive and extremely dangerous. A pos- 
sible alternative to HNO3-HF pickling was 
reported in a series of research articles 
(22-25) . This research resulted in a 
technique (24) that used an applied po- 
tential to dissolve the chromium-depleted 
zone while minimizing the dissolution of 
the base material in an H 2 S0 4 bath. The 
researchers claim that a proper choice of 
potential can optimize the pickling of 
stainless steels while using the less 
hazardous H 2 S0 4 solution. However, this 
technique does not produce as reflective 
a surface as produced in free (not poten- 
tial controlled) pickling in HNO3-HF mix- 
tures. The researchers also did not in- 
vestigate the effects of iron, chromium, 
and nickel buildup in the bath on the po- 
tential controlled pickling or the quali- 
ty of the pickled surface. As in HNO3-HF 
solutions , these impurities could affect 
the electrochemical reactions and the 
rate of pickling, and could even lead 
to pitting or increased intergranular 
attack. 



11 



Other pickling solutions have been ex- 
amined in previous research efforts. In 
one study (33) the solutions were select- 
ed for their ability to dissolve the bulk 
steel. This research reported on the re- 
lationship of the composition of the 
pickling bath to the dissolution rate of 
304 stainless steel. Using various com- 
binations of H 2 S0 4 , HC1, HN0 3 , and NaN0 3 , 
it was shown that HC1 and HNO3 have about 
an equal effect on the dissolution rate 
of scale-free 304 SS, while H 2 S0 4 has a 
much lower effect and NaN03 has a much 
greater effect (NaN0 3 >HCl«HN0 3 >H 2 S0 4 ) . 
The results of this study assumed that 
each chemical in the pickling bath can 
operate individually with no synergistic 
effect. However, the same group of chem- 
icals produced different results on an- 
nealed and oxide-covered 304 stainless 
steel. The NaN0 3 solution had the slow- 
est pickling rate, while HC1 had the 
fastest pickling rate. Both H 2 S0 4 and 
HNO3 had intermediate rates. This sug- 
gests that while dissolution may be im- 
portant in the mechanism of pickling, 
dissolution of the bulk alloy may not be 
critical. When solutions containing HF 
were tested, results showed that it had 
as strong an effect on pickling rate as 
HC1. HC1 is usually not used because the 
FeCl 3 formed generally promotes pitting 
of the stainless steel. No studies of 
the effect of temperature on the pickling 
of stainless steels were found in this 
search of the literature. 

BULK ALLOY DISSOLUTION 

Solution Composition 

The aforementioned report (33) suggest- 
ed that dissolution of the bulk steel may 
not be critical in determining the rate 
of pickling of stainless steels. The 
dissolution rate of the bulk steel does, 
however, control the amount of bulk steel 
lost to the pickling solution and to some 
extent the amount of dissolved metal 
species in solution. The factors that 
could affect this dissolution rate are 
temperature, acid concentration, dis- 
solved metal concentration, and agita- 
tion. Since many dissolution reactions 
exhibit some dependence on convection and 



diffusion, agitation of the solution 
should be important. A study (34) was 
done that showed that agitation, as simu- 
lated by a rotating disk electrode, sig- 
nificantly affects the dissolution of a 
304-type stainless steel (Khl8N10T) in 
HNO3 + NaCl solutions. Dissolution rates 
for samples rotated at to 300 rpm were 
not affected, whereas rates increased 
steadily with increasing rotation speed 
from 300 to 10,000 rpm. The investigator 
assumed that the agitation affected main- 
ly the cathodic reduction of nitric to 
nitrous acid, but offers no evidence. 
Another investigator has shown (35) , to 
the contrary, that the anodic reaction of 
nitric acid on platinum is diffusion de- 
pendent while the cathodic reaction is 
only reaction dependent. 

Acid Concentration 

The effect of HN0 3 concentration on the 
dissolution of austenitic stainless 
steels is minimal over a range of concen- 
trations and temperatures that would be 
used in pickling solutions. Isocorrosion 
diagrams (7_, p. 243) for 18-8S4 steels 
show that the corrosion rate ranges from 
to 125 um/yr for HN0 3 concentrations up 
to 50 wt pet from 30° C to the boiling 
point. The dissolution behavior of sev- 
eral of the more popular austenitic 
stainless steels and of some iron- 
chromium-nickel alloys has been thorough- 
ly reviewed ( 36 ) elsewhere. The results 
simply show that HNO3 solutions (<50 pet) 
do not rapidly dissolve austenitic stain- 
less steels. 

Mixed Acids 

To dissolve the austenitic stainless 
steels , a mixed acid is usually consid- 
ered. Mixtures of HNO3 and H 2 S0 4 in- 
crease the dissolution rate of the steels 
by about a factor of four over HNO3 alone 
(7^, p. 243); however, this combination is 
rarely used in pickling operations. The 
most commonly used solutions for pick- 
ling austenitic stainless steels contain 
HNO3 and HF. The effect of this mixed 

^"S" means lower carbon content to pre- 
vent carbide precipitation. 



12 



acid on various austenitic steels has 
been extensively studied and adequately 
reviewed (37) . HF significantly in- 
creases the rate of dissolution of aus- 
tenitic steels in HN0 3 solutions. For 
example, as little as 0.5 wt pet HF in 
18 wt pet HN0 3 can increase the dissolu- 
tion rate of 304 stainless steel by two 
orders of magnitude at 60° C and by al- 
most three orders of magnitude at 80° C 
(37) . This corrosion rate is approxi- 
mately 15,000 um/yr at 80° C. 

Temperature 

The above results suggest that tempera- 
ture has a significant effect on the dis- 
solution of austenitic stainless steels. 
A study ( 37 ) conducted on 309SCb 5 stain- 
less steel indicated that a temperature 
change from 20° to 100° C increased the 
dissolution rate in HNO3-HF solutions by 
over two orders of magnitude. This re- 
sponse to increasing temperature was 
shown to be true regardless of solution 
compositions from 4.5 wt pet HNO3 + 0.2 
wt pet HF to 27 wt pet HNO3 + 2 wt pet 
HF. These results imply that the activa- 
tion energy for dissolution is not af- 
fected by solution composition, although 
the absolute magnitude of that dissolu- 
tion rate was found to be affected by so- 
lution composition. 



CHROMIUM-DEPLETED ZONE DISSOLUTION 

The dissolution behavior of the 
chromium-depleted zone should be affected 
by the same factors as those affecting 
the bulk steel: temperature, acid con- 
centration, and agitation. Dissolution 
rates should be higher than those for the 
bulk steel because of the reduced chromi- 
um and nickel contents. Direct studies 
of the dissolution behavior of this de- 
pleted zone have not been conducted be- 
cause of the extreme thinness of this 
zone. Indirect studies, however, have 
been done by fabricating alloys that sim- 
ulate different regions of the depleted 
zone. A study (38) of a series of iron- 
chromium-nickel alloys (2 to 18 wt pet 
Cr) showed that the dissolution rate in 
sulfuric acid was high for very low con- 
centrations of chromium. Surprisingly, 
however, the dissolution rate passed 
through a minimum at 12 wt pet Cr where 
the dissolution rate was only 40 pet of 
that for a 19 wt pet Cr alloy. Studies 
of similar alloys in HNO3-HF would be 
very important for developing an under- 
standing of the pickling of stainless 
steels. Equally important would be stud- 
ies of the effect of acid concentration, 
temperature, and agitation on the disso- 
lution behavior of these chromium- 
depleted alloys. 



RESEARCH NEEDS 



Based on this review of the literature 
on the pickling of stainless steels, the 
following areas have been identified as 
needing further study: 



3. The effect of the various condi- 
tioning techniques on the structure and 
composition of the scale related to its 
effect on pickling rate. 



1. The effect of hot working on the 
grain size of metal and subsequent oxide 
scale composition. 

2. The effect of annealing parameters 
such as furnace environment and time at 
temperature on the annealing scale and 
ultimately on the pickling rate. 

S" means lower carbon to prevent car- 
bide precipitation, and "Cb" means nio- 
bium (columbium) added to prevent carbide 
precipitation . 



4. The dissolution behavior of bulk 
steels and the chromium-depleted zone of 
bulk steels, and the effects of the pick- 
le bath variables, acid concentration, 
temperature, and agitation, on the disso- 
lution rate. 

In conclusion, additional studies in 
any one of the above four areas will con- 
tribute to the understanding of the pick- 
ling process. However, knowledge from 
all four areas is necessary to develop 
the relationships needed to quantify the 



13 



pickling process. This quantification 
should lead to an improvement in the ef- 
ficiency of pickling, a reduced cost to 
process stainless steels, and minimizing 
the loss of critical metals such as nick- 
el and chromium. Laboratory studies are 



presently being done to understand the 
effect of acid concentration, tempera- 
ture, and dissolved metal concentration 
on the pickling of 304 and 430 stainless 
steels. 



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14 



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15 



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